The present invention relates generally to a longitudinally adjustable leg assembly, which is adapted to be attached to the underside of a table top.
Many people have work assignments which cause them to sit still all day and, for instance, work at a computer. It is therefore becoming more and more important to create a workplace that affords a good working posture, which requires that the height of working table and work surface be variable.
Consequently it has become more and more common to have vertically adjustable tables which can be adjusted so that the working posture will be as good as possible and moreover makes it possible for several people to share the workplace. Vertically adjustable tables which make it possible to vary the user""s working posture between sitting and standing result in an additional improvement of the workplace since a varied working posture wears less on the user""s body.
The vertical adjustability is usually controlled by adjusting the length of the legs of the table, and it is advantageous if this does not have to be made manually, but instead can be effected using e.g. an electric motor.
A problem with electric motors for adjusting the length of table legs is, however, that parts of the electric motor are rapidly worn. When the motor drives the leg to a limit position where the leg is at its longest or shortest, the movable parts of the motor are subjected to heavy wear. This wear arises when the leg in the limit position stops moving and the motor is exposed to a high load since it continues trying to drive the leg past the limit position.
One way of controlling the maximum and minimum height of a table leg is shown in EP 0 283 103. An electric motor is incorporated in the table leg for driving a screw spindle. The electric motor is connected to the lower free end of said screw spindle and suspended freely thereon in the lower leg part. The motor is movable up and down in the lower leg. Microswitches are arranged on a spindle nut for cooperation with the motor. The solution requires a number of leads arranged between movable parts inside the leg.
An object of the present invention is to minimise wear on an electric motor for a longitudinally adjustable leg assembly and, thus, increase the service life of the motor. A further object of the invention is to provide a limit switch for an electric motor, which performs the setting of the length of a leg assembly. Still another object is to provide a limit switch which is of a simple construction and whose function does not prevent the motor from setting the length of the leg within a permissible range.
According to the invention, the objects are achieved by means of a longitudinally adjustable leg assembly, comprising a holder which is adapted to be fixed to the underside of a table top, and first and second elongate carrier elements which are directed away from the holder and cooperate by threaded engagement. The first carrier element is mounted rotatably but axially immovably in the holder and has preferably an external thread. The second carrier element is preferably a tube having an internal thread portion at an end enclosing the first carrier element, in threaded engagement with the external thread of the first carrier element. The leg assembly further comprises an electric motor for rotating the first carrier element, and a device for interrupting the current to the motor when the leg assembly is in a limit position of maximum or minimum length and a limit position wire. The device for interrupting comprises two switches and an actuator, which is adapted to actuate one of the two switches when the leg assembly is in the limit position of maximum length and the other of the two switches when the leg assembly is in the limit position of minimum length. The limit position wire is attached to the actuator for controlling the same when the leg assembly is in its limit position of maximum length.
By means of the device for interrupting the current, it is ensured that the motor does not try to drive the leg assembly past a limit position, thus preventing unnecessary and heavy wear on the motor and its components. Moreover, the device for interrupting is designed so that a situation when both switches are switched off cannot arise since only one actuator is available. This prevents a position where the leg assembly gets stuck on a certain level. Moreover, the device for interrupting has but a few components, which results in a simple device with a small risk of jamming. The limit position wire is fixed to the actuator for controlling the same, i.e. its actuation of the switch, when the leg assembly is in its limit position of maximum length. This means that the actuator is moved when it is to actuate one of the switches. The limit position wire makes it possible for the actuator to be moved when the leg assembly has reached its limit position of maximum length.
The device for interrupting with its two switches is preferably arranged at an upper end of the first carrier element.
As a result, there is no problem with wires extending along the carrier elements of the leg assembly. In fact, these move when the length of the leg assembly is being set, which would make it difficult to mount wires to the switches in a safe manner. The position of the witches also implies that the device for interrupting an be placed in such manner that the switches are not moved relative to the table top when the length of the leg assembly is being set. As a result, there is no need for wires that accompany the motions of the carrier elements, which would be still more complicated. Moreover the motor is often arranged close the upper end of the first carrier element since this is driven by the motor. This means that short wires can be used, which is always advantageous.
The limit position wire has advantageously a lower abutment, with which the second carrier element interacts for controlling of the actuator when the leg assembly is in its limit position of maximum length.
As a result, power transmission from the second carrier element to the limit position wire can easily be obtained when the leg assembly has reached this limit position. The limit position wire is then actuated by the second carrier element so as to control the actuator to actuate one of the switches.
In the case with two carrier elements, the second carrier element can advantageously be arranged to interact, directly or indirectly, with the actuator for controlling the same when the leg assembly is in its limit position of minimum length.
As a result, it will be possible to arrange the switches close to each other. Nor does the user have to consider in the leg assembly an actuator which moves when the length of the leg assembly is being adjusted in the areas outside the limit positions.
The actuator is suitably spring actuated towards a neutral position where none of the switches is actuated.
This requires that the actuator be subjected to a certain degree of minimum force so as to be moved, thereby actuating one of the switches. This ensures that none of the switches is actuated owing to some minor interference and that a good function is maintained. Only when the leg assembly reaches a limit position, the actuator is subjected to a sufficient degree of force for one of the switches to be actuated.
The device for interrupting preferably comprises two springs, which from one direction each urge the actuator towards the neutral position where none of the switches is actuated.
This results in spring forces from two opposite directions in order to hold the actuator in a neutral position. Thus, essentially the same force is required to move the actuator in each direction. This spring action also causes the actuator to be moved to the neutral position as soon as the leg assembly is being moved from a limit position. This means that the leg assembly can all the time be lengthened or shortened when not in a limit position. This also eliminates the risk that the length of the leg assembly is to be changed significantly from the length in a limit position before the length can be changed back to the same limit position. This could be necessary if the actuator is not immediately moved to its neutral position when the leg assembly leaves the limit position.
Each switch prevents, when actuated, the motor from being run in a direction equivalent to that in which the leg assembly has reached its limit position.
This means that the motor can always be run in the opposite direction, i.e. in the direction that moves the leg assembly away from the limit position.
The leg assembly advantageously comprises a third elongate and preferably tubular carrier element, which is directed away from the holder, is arranged non-rotatably relative to the holder and has a thread portion in threaded engagement with a thread of the second carrier element. Preferably, the thread portion of the third carrier element is an internal thread at an end enclosing the second carrier element, in threaded engagement with an external thread along the entire extent of the second carrier element. A second limit position wire interacts, with the aid of a shifter, with the first limit position wire for controlling the actuator when the leg assembly is in its limit position of maximum length.
The second limit position wire is suitably fixed in the shifter for controlling the actuator by means of the limit position wire fixed in the actuator when the leg assembly is in its limit position of maximum length. Moreover, the second limit position wire can have a lower abutment, with which the third carrier element interacts to control the actuator when the leg assembly is in its limit position of maximum length. Finally the third carrier element can be arranged to interact, preferably with the aid of the shifter, with the actuator for controlling the same when the leg assembly is in its limit position of minimum length.
This results in limit switching for a leg assembly with three carrier elements. This leg assembly has a greater range of possible lengths, but at the same time higher demands are placed on the limit position function since the switch cannot be actuated until all three carrier elements have been extended or retracted. This is solved by means of the second limit position wire which interacts with a shifter to actuate the first limit position wire when the leg has been fully extended, i.e. reached its maximum length.